Sodium chlorate is generally prepared by the electrolysis of sodium chloride wherein the sodium chloride is electrolyzed to produce chlorine, sodium hydroxide and hydrogen. The chlorine and sodium hydroxide are immediately reacted to form sodium hypochlorite, which is then converted to chlorate and chloride under controlled conditions of pH and temperature.
Thus, within the electrolytic system, sodium chloride is, in effect, combined with water to form sodium chlorate and hydrogen gas. The electrolysis takes place typically at 60.degree. C. to 90.degree. C. in electrolytic cells comprising anodes, which may be a precious metal or metal oxide coated titanium, and, typically, steel cathodes.
In another chemical process, chlorine and caustic soda are prepared in an electrolytic cell, which contains a membrane to prevent chlorine and caustic soda reacting and the separated chemicals removed.
The sodium chloride salt used to prepare the brine for electrolysis to sodium chlorate generally contains impurities which, depending on the nature of the impurity and production techniques employed, can give rise to plant operational problems familiar to those skilled in the art. The means of controlling these impurities are varied and include, purging them out of the system into alternative processes or to the drain, precipitation by conversion to insoluble salts, crystallization or ion exchange treatment. The control of anionic impurities presents more complex problems than that of cationic impurities.
Sulphate ion is a common ingredient in commercial salt. When such salt is used directly, or in the form of a brine solution, and specific steps are not taken to remove the sulphate, the sulphate enters the electrolytic system. Sulphate ion maintains its identity under the conditions in the electrolytic system and thus accumulates and progressively increases in concentration in the system unless removed in some manner. In chlorate plants producing a liquor product, the sulphate ion will leave with the product liquor. In plants producing only crystalline chlorate, the sulphate remains in the mother liquor after the crystallization of the chlorate, and is recycled to the cells. Over time, the concentration of sulphate ion will increase and adversely affect electrolysis and cause operational problems due to localized precipitation in the electrolytic cells. Within the chloralkali circuit the sodium sulphate will concentrate and adversely effect the membrane, which divides the anolyte (brine) from the catholyte (caustic soda).
It is industrially desirable that sodium sulphate levels in concentrated brine, e.g., 300 g/l NaCl be reduced to at least 20 g/l in chlorate production and 10 g/l in chloralkali production.
U.S. Pat. No. 4,702,805, Burkell and Warren, issued Oct. 27, 1987, describes an improved method for the control of sulphate in an electrolyte stream in a crystalline chlorate plant, whereby the sulphate is crystallized out. In the production of crystalline sodium chlorate according to U.S. Pat. No. 4,702,805, sodium chlorate is crystallized from a sodium chlorate rich liquor and the crystals are removed to provide a mother liquor comprising principally sodium chlorate and sodium chloride, together with other components including sulphate and dichromate ions. A portion of the mother liquor is cooled to a temperature to effect crystallization of a portion of the sulphate as sodium sulphate in admixture with sodium chlorate. The crystallized admixture is removed and the resulting spent mother liquor is recycled to the electrolytic process.
It has been found subsequently, that the crystallized admixture of sulphate and chlorate obtained from typical commercial liquors according to the process of U.S. Pat. No. 4,702,805 may be discoloured yellow owing to the unexpected occlusion of a chromium component in the crystals. The discolouration cannot be removed by washing the separated admixture with liquors in which the crystallized sulphate and chlorate are insoluble. It will be appreciated that the presence of chromium in such a sulphate product is detrimental in subsequent utilization of this product and, thus, this represents a limitation to the process as taught in U.S. Pat. No. 4,702,805.
U.S. Pat. No. 4,636,376 -- Maloney and Carbaugh, issued Jan. 13, 1987, discloses removing sulphate from aqueous chromate-containing sodium chlorate liquor without simultaneous removal of significant quantities of chromate. The chromate and sulphate-containing chlorate liquor having a pH in the range of about 2.0 to about 6.0 is treated with a calcium-containing material at a temperature of between about 40.degree. C. and 95.degree. C., for between 2 and 24 hours to form a sulphate-containing precipitate. The precipitate is predominantly glauberite, Na.sub.2 Ca(SO.sub.4).sub.2. However, the addition of calcium cations requires the additional expense and effort of the treatment and removal of all excess calcium ions. It is known that calcium ions may form an unwanted deposit on the cathodes which increases the electrical resistance of the cells and adds to operating costs. It is, typically, necessary to remove calcium ions by means of ion exchange resins.
U.S. Pat. No. 5,093,089--Alford and Mok, issued Mar. 3,1992 describes an improved version of the selective crystallization process of aforesaid U.S. Pat. No. 4,702,805, wherein process conditions are selected to provide precipitation of sulphate substantially free of chromium contaminant.
Typically, organic anion exchange resins have a low selectivity for sulphate anions in the presence of a large excess of chloride ions. U.S. Pat. No. 4,415,677 describes a sulphate ion absorption method, but which method has disadvantages.
The method consists of removing sulphate ions from brine by a macroporous ion exchange resin composite having polymeric zirconium hydrous oxide contained in a vessel. This method is not economical because the efficiency is low and a large amount of expensive cation exchange resin is required for carrying polymeric zirconium hydrous oxide. Further, the polymeric zirconium hydrous oxide adsorbing Sulphate ions comes into contact with acidic brine containing sulphate ions, resulting in loss of polymeric zirconium hydrous oxide due to acid-induced dissolution. Soluble zirconyl ions precipitates as hydroxide in the lower portion of the vessel to clog flow paths.
U.S. Pat. No. 4,556,463--Minz and Vajna issued Dec. 3, 1984, describes a process to decrease sulphate concentration levels in brine solutions using an organic ion exchange material with brine streams under carefully controlled dilutive conditions.
U.S. Pat. No. 5,071,563--Shiga et al, issued Dec. 10, 1991, describes the selective adsorption of sulphate anions from brine solutions using zirconium hydrous oxide slurry under acidic conditions. The ion exchange compound may be regenerated by treatment with alkali.
Japanese Patent Kokai No. 04321514-A, published Nov. 11, 1992 to Kaneka Corporation describes the selective adsorption of sulphate anions from brine solutions using cerium hydroxide slurry under acidic conditions. The ion exchange compound may be regenerated by treatment with alkali.
Japanese Patent Kokai No. 04338110-A-- Kaneka Corporation, published Nov. 25, 1992 describes the selective adsorption of sulphate anions from brine solutions using titanium hydrous oxide slurry under acidic conditions. The ion exchange compound may be regenerated by treatment with alkali.
Japanese Patent Kokai No. 04334553-A-- Kaneka, published Nov. 11, 1992 describes the removal of sulphate ions from brine using ion-adsorbing cakes in a slurry.
There still remains, however, a need for an improved, cost-effective, practical method for the removal of sulphate ions from alkali metal halide solutions, particularly, from sodium chloride solutions used in the electrolytic production of sodium chlorate and chlorine/caustic soda.